Chemically stable translucent apatite glass ceramic

ABSTRACT

An apatite glass ceramic is described which is characterized by good chemical stability, a low expansion coefficient and high translucence, and is particularly suitable, by itself or together with glasses or other glass ceramics, as a veneering material for ceramic dental restorations.

This appln claims the benefit of U.S. Provisional No. 60/064,015 filedNov. 3, 1997.

The invention relates to a chemically stable, translucent apatite glassceramic which is particularly suitable for use in restorative dentistryand above all for coating or veneering of dental restorations, such asbridges or crowns.

Apatite glass ceramics are known from the prior art. They are usuallyemployed as bioactive materials for replacing bone in human medicine, oras the main component of glass ionomer cements in dentistry.

In the case of bioactive materials for bone replacement, they have veryhigh CaO and P₂O₅ contents, however, in order to achieve bioactivity,i.e. the direct growing together of glass ceramic and living bone.

A glass ceramic implantation material is known from DE-A-40 20 893 whichhas apatite crystals but also contains very large quantities of CaO inorder to achieve bioactivity.

Glass ceramics for glass ionomer cements also have high CaO contents andmostly also high fluoride ion contents, in order to obtain the desiredhigh level of ion release in the oral medium.

These two types of apatite glass ceramics are white-opaque, however, andhave a high level of ion release and/or bioactivity, so they are notsuitable for restorative dentistry.

An apatite glass ceramic for restorative dentistry must have opticalproperties such as translucence and colour which are similar to those ofthe natural tooth. A material which is impervious to light, i.e. opaque,is not suitable for this purpose. Moreover, bioactivity or a high levelof ion release is undesirable; rather, a high degree of chemicalstability is required which should even exceed that of the naturaltooth.

In known apatite-containing glass ceramics for restorative dentistry,the main crystal phase is regularly formed not by apatite but by leuciteor mullite. This is undesirable, however, since these types of crystalsmake it difficult, inter alia to imitate the optical properties of thenatural tooth material composed primarily of needle-shaped apatite.

EP-A-0 690 030 discloses leucite-containing phosphosilicate glassceramics which may be used in dental engineering. In view of the leucitecontent, however, they have very high thermal expansion coefficients sothey are not -suitable for the coating of materials with low expansioncoefficients, such as lithium disilicate glass ceramics.

Moreover, an apatite glass ceramic containing mullite as a furthercrystal phase is described by A. Clifford and R. Hill (Journal ofNon-Crystalline Solids 196 (1996) 346-351). The high mullite contentbrings about only low translucence.

Apatite-containing glass ceramics are disclosed by S. Hobo et al.(Quintessence International 2 (1985) 135-141) and Wakasa et al. (J. OralRehabil. 17 (1990) 461-472 and J. Mat. Sci. Lett. 11 (1992) 339-340) forrestorative tooth replacement. Said glass ceramics have high CaO andP₂O₅ contents, however, so they show only poor chemical stability.Moreover, the apatite crystals in these glass ceramics do not have aneedle-shaped morphology.

Moreover, DE-A-34 35 348 describes apatite-containing glass ceramics forthe production of dental crowns. The glass ceramics, however, contain noAl₂O₃ at all and very large quantities of CaO, for which reason theyhave a high tendency to ion exchange and consequently only poor chemicalstability. In addition, the apatite crystals do not have theneedle-shaped morphology which is characteristic of apatite crystals ofnatural tooth material.

Glass ceramics with good chemical stability are disclosed in EP-A-0 695726 as alkali-zinc-silicate glass ceramics. The disadvantage of saidglass ceramics, however, is that they contain leucite rather thanapatite as the crystal phase. As a result of the high expansioncoefficient of leucite, the glass ceramics are therefore usuallyunsuitable as coatings for substrates with low expansion coefficients,such as, in particular, lithium disilicate glass ceramics. The glassceramic also necessarily contains ZnO in order to achieve good chemicalstability.

The object of the invention, therefore, is to provide an apatite glassceramic which resembles natural tooth material in terms of its opticalproperties and, in particular, its high translucence, and containsapatite crystals which have a greater chemical stability than thecarbonate-apatite crystals of natural tooth material and hence conferexcellent chemical stability on the glass ceramic. Moreover, the apatiteglass ceramic should preferably contain apatite crystals with a similarmorphology to that of the apatite crystals of natural tooth material andhave a low thermal expansion coefficient and should therefore beparticularly suitable as a dental material and above all as a coating orveneer for dental restorations, such as crowns or bridges, made oflithium disilicate.

Surprisingly, said object is achieved by the chemically stable,translucent apatite glass ceramic according to the present invention.

The invention also provides the process for the production of theapatite glass ceramic, a dental material as well as dental uses andshaped dental products.

The apatite glass ceramic according to the invention is characterised inthat it contains CaO, P₂O₅ and F in a molar ratio of:

CaO:P₂O₅:F 1:0.020 to 1.5:0.03 to 4.2

and contains apatite crystals as the main crystal phase.

The molar ratio of CaO:P₂O₅:F in the glass ceramic is preferably 1:0.1to 0.5:0.1 to 1.0 since this leads to particularly stable glassceramics.

Surprisingly, it has become apparent that by adjusting the molar ratioof CaO to P₂O₅ to F in the stated manner, apatite glass ceramics areobtained which show an increased chemical stability compared withconventional apatite glass ceramics.

In order to quantify the chemical stability, the test method accordingto ISO specification 6872:1995 described for glass ceramics was carriedout, in which the resistance to aqueous acetic acid solution isdetermined by measuring the loss of mass in μg/cm² that occurs. Theessential stages of this method are given in the Examples.

The apatite glass ceramic according to the invention usually exhibits aloss of mass of less than 200 and preferably less than 100 μg/cm².Particularly preferred glass ceramics have a loss of mass of less than60 μg/cm².

Advantageous apatite glass ceramic according to the invention is alsocharacterised in that it contains at least one of the followingcomponents:

Component Wt. % SiO₂ 45.0 to 70.0 Al₂O₃  5.0 to 22.0 P₂O₅ 0.5 to 6.5 K₂O3.0 to 8.5 Na₂O  4.0 to 13.0 CaO  1.5 to 11.0 F 0.1 to 2.5

The glass ceramic according to the invention may additionally contain atleast one of the following components:

Component Wt. % B₂O₃ 0 to 8.0 La₂O₃ 0 to 5.0 Li₂O 0 to 5.0 BaO 0 to 5.0MgO 0 to 5.0 ZnO 0 to 5.0 SrO 0 to 7.0 TiO₂ 0 to 4.0 ZrO₂ 0 to 4.0 CeO₂0 to 3.0

The lower limits for those additional components are usually 0.05 wt. %.

Preferred quantity ranges exist for the individual components of theapatite glass ceramic according to the invention. Unless otherwisespecified, these may be chosen independently of one another and are asfollows:

Component Wt. % SiO₂ 50.0 to 68.0 Al₂O₃  7.0 to 21.0 P₂O₅ 0.5 to 4.0 K₂O4.0 to 8.0 Na₂O  4.0 to 11.0 CaO 2.0 to 8.0 F 0.2 to 2.0 B₂O₃ 0.2 to 4.0La₂O₃   0 to 3.0 Li₂O   0 to 3.0 BaO   0 to 4.0 MgO   0 to 4.0 ZnO   0to 4.0 SrO   0 to 5.0 TiO₂ + ZrO₂ 0.2 to 5.0 CeO₂   0 to 2.0

Particularly preferred quantity ranges for the individual components ofthe apatite glass ceramic according to the invention are as follows andthese may be chosen independently of one another:

Component Wt. % SiO₂ 54.0 to 65.0 Al₂O₃  8.0 to 21.0 P₂O₅ 0.5 to 3.5 K₂O5.0 to 8.0 Na₂O  6.0 to 11.0 CaO 2.0 to 6.0 F 0.3 to 1.5 B₂O₃ 0.2 to 3.0La₂O₃   0 to 2.0 Li₂O   0 to 2.0 BaO   0 to 3.0 MgO   0 to 3.0 ZnO   0to 3.0 SrO   0 to 4.0 TiO₂ 0.5 to 2.0 ZrO₂ 0.5 to 3.0 CeO₂ 0.1 to 1.5

All the above-mentioned quantities in wt. % relate to the glass ceramic.

The glass ceramic according to the invention may also contain e.g.conventional colour components for matching the colour of a patient'snatural tooth material.

It was ascertained by scanning electron microscope and X-ray diffractionanalyses that apatite, such as hydroxy and/or fluoroapatite, forms themain crystal phase in the glass ceramic. The apatite crystals arepreferably hexagonal and in particular needle-shaped. The apatitecrystals are preferably smaller than 35 μm in their greatest extension,particularly smaller than 15 μm, and in particular preference smallerthan 5 μm.

The optical properties of the glass ceramic are controlled by means ofthe precipitated apatite crystals which preferably resemble thecarbonate-apatite crystals of natural tooth material in terms of theirappearance. It is thus possible to produce a glass ceramic with anappearance which corresponds to that of the dentine or enamel of thetooth. At the same time, an optical depth is achieved in the glassceramic which is not possible by means of other types of crystals.

The glass ceramic according to the invention is characterised not onlyby very good chemical stability but also by translucence. In order toquantify the translucence, the CR value was determined according to themethod described in the Examples. The CR value, also known as thecontrast ratio, indicates the ratio of light reflection of a specimen ofthe glass ceramic on a black background to the measurement of the lightreflection of the specimen on a white background, and thus serves as ameasure of the translucence of a material. The CR value is defined bythe following formula:

CR=Y _(b) /Y _(w)

where

CR=contrast ratio

Y_(b)=light reflection of the specimen on a black background, and

Y_(w)=light reflection of the specimen on a white background.

The CR value always lies in the range from 0 to 1, where CR=0 stands foran opacity of 0% and consequently a completely translucent material, andCR=1 stands for an opacity of 100% and consequently a completely opaquematerial, i.e. one which is impervious to light.

The glass ceramic according to the invention usually has a CR value of 0to 0.9 and preferably 0.1 to 0.75.

A further particular advantage of the glass ceramic according to theinvention is that, due to its particular molar ratio of CaO to P₂O₅ to Fin combination with the precipitated apatite crystals, good chemicalstability is achieved without ZnO necessarily having to be present.

It is presumed that this stability is also attributable to the very highdegree of crystallinity and to the preferential formation of hydroxy andfluoroapatite. The stability of the precipitated fluoro orhydroxyapatite crystals is higher than that of the rather unstablecarbonate-apatite, which is present in natural tooth material.

It should also be pointed out that the glass ceramic may be produced inthe B₂O₃-free form. The advantage of adding B₂O₃, however, is that theentire sintering behaviour of the glass ceramic is improved andsintering can take place in the preferred temperature range of 650° C.to 1050° C.

The apatite glass ceramic usually has a very low thermal expansioncoefficient of 6.0 to 12.0×10⁻⁶K⁻¹, measured in the temperature rangefrom 100° C. to 400° C.

In order to produce the apatite glass ceramic according to theinvention,

a) a starting glass containing the necessary components is melted attemperatures of 1200° C. to 1650° C.,

b) the glass melt obtained is poured into water with the formation ofglass granules,

c) the glass granules are optionally comminuted to a glass powder withan average particle size of 1 to 450 μm, based on the number ofparticles, and

d) the glass granules or glass powder is subjected to a heat treatmentat temperatures of more than 900° C. and up to 1200° C. for a period of30 minutes to 6 hours.

In stage (a), a starting glass is first melted by intimately mixingsuitable starting materials, such as carbonates, oxides and fluorides,and heating them to the given temperature.

In stage (b), the glass melt obtained is then quenched by being pouredinto water and is thereby converted to glass granules. This procedure isusually also referred to as fritting.

Optionally, the glass granules are then comminuted in stage (c) andground, particularly with conventional mills, to the desired particlesize. The glass powder obtained preferably has an average particle sizeof 1 to 450 μm, based on the number of particles.

In stage (d), the glass granules or optionally the glass powder undergoa heat treatment at temperatures of more than 900° C. to 1200° C. for aperiod of 30 minutes to 6 hours, preferably 30 minutes to 3 hours. Atemperature of more than 900° C. is required since the development ofthe apatite crystals in the desired form and quantity does not takeplace at lower temperatures.

Volume crystallisation takes place during the heat treatment. This leadsto a homogeneous distribution of the apatite crystals throughout theglass ceramic, in contrast to leucite-crystallisation, which can onlyoccur on the internal surfaces of a glass powder.

The process of glass fritting described in stage (b) is responsible forfreezing a glass structure with very small (<100 nm) droplet-shapedprecipitates which are extremely densely packed and finely distributed.Even under the scanning electron microscope with a 30,000 foldmagnification, a residual glass matrix can no longer be detected. It isassumed that the apatite crystallisation taking place during thesubsequent heat treatment proceeds via these precipitates which cantherefore be regarded as primary nuclei.

It was ascertained by scanning electron microscopy and X-ray diffractionanalyses that apatite, preferably fluoroapatite, forms the main crystalphase. The size of the crystals obtained can be controlled by thetemperature selected and the duration of the heat treatment. In additionto the apatite crystals, further crystal phases may be formed dependingon the chemical composition of the starting glass used. In addition tothe various crystal phases, microheterogeneous demixing regions, i.e.various glass phases, may also be present. These regions can beidentified under the scanning electron microscope as smallmicroheterogeneous droplet glass phases about 20 to 400 nm in size. Thedroplet glass phases occurring, together with the crystals, influencethe optical properties of the glass ceramics according to the invention,such as opalescence and translucence.

Surprisingly, the optical properties of the apatite glass ceramicaccording to the invention may be adjusted from glassy transparent towhitish cloudy. This is absolutely vital for use as dental material orcomponent thereof in order to be able to produce all the various formsof the natural tooth in a reproducible manner. The fine apatite crystalsin the microstructure of the glass ceramic preferred according to theinvention bring about a very great similarity to the natural tooth interms of optical appearance and structure.

The apatite glass ceramic according to the invention is therefore usedparticularly as a dental material and preferably as a component ofdental material.

When apatite glass ceramic is used as a component of dental material, itis possible, by a suitable choice of its composition and of the type ofother components, to obtain dental materials with good chemicalstability in which important properties, such as processing temperature,optical properties and thermal expansion coefficient, are matchedexactly to the respective requirements. This is often not possible withpure glass ceramic.

A combination of the desired properties may be obtained with the apatiteglass ceramic according to the invention by mixing it with glassesand/or other glass ceramics. It is preferable in this case that thedental material contains 10 to 90 wt. % of the apatite glass ceramic.

It is in particular possible to use the glass ceramic according to theinvention as a means to modify the optical properties of glasses andother glass ceramics. In case of a dental ceramic it is a goal toachieve balance between translucence and lightness, which closelyresembles the natural teeth. A satisfactory dental restoration mustsimultaneously have a bright appearance and a high translucence.

Upon use of conventional opacifiers, such as SnO₂, this cannot beobtained. If the lightness is satisfactory, then the translucence is toolow to match the properties of natural teeth.

By using the apatite glass-ceramic according to the invention asopacifier having cristalls of a size of generally up to 15 μm andparticularly up to 5 μm a lightness and translucence similar to that ofnatural teeth can surprisingly be obtained.

The dental material according to the invention preferably contains, inaddition to the apatite glass ceramic, at least one glass and/or glassceramic of the systems comprising alkali-silicate, alkali-alkaline earthsilicate, alkali-aluminosilicate, alkali-zinc-borosilicate,phosphosilicate or alumino-fluoro-borosilicate. Preferred glass ceramicsand glasses of this kind are given below, the details in wt. % relatingto the glass ceramic in question or the glass in question.

Leucite-containing phosphosilicate glass ceramic having the composition:

SiO₂ 49.0-57.5 wt. %, Al₂O₃ 11.4-21.0 wt. %, P₂O₅ 0.5-5.5 wt. %, CaO2.5-11.5 wt. %, K₂O 9.0-22.5 wt. %, Na₂O 1.0-9.5 wt. %, Li₂O 0-2.5 wt.%, B₂O₃ 0-2.0 wt. %, TiO₂ 0-3.0 wt. %, ZrO₂ 0.8-8.5 wt. %, CeO₂ 0-3.0wt. %, F 0.25-2.5 wt. %, La₂O₃ 0-3.0 wt. %, ZnO 0-3.0 wt. %, BaO 0-3.0wt. %, MgO 0-3.0 wt. % and SrO 0-3.0 wt. %.

Opalescent glasses having the composition:

SiO₂ 48.0-66.0 wt. %, B₂O₃ 0-1.0 wt. %, Me(III)₂O₃ 5.8-20.0 wt. %,Me(I)₂O 6.0-22.0 wt. %, Me(II)O 3.5-16.0 wt. %, Me(IV)O₂ 0.5-10.0 wt. %,P₂O₅ 0.5-5.0 wt. %, CeO₂ 0-3.0 wt. %, wherein the quantity of Me(III)₂O₃is formed by 5.8-20.0 wt. % of Al₂O₃ and 0-6.0 wt. % of La₂O₃; thequantity of Me(I)₂O is formed by 3.0-15.0 wt. % of K₂O, 3.0-12.0 wt. %of Na₂O and 0-2.5 wt. % of Li₂O; the quantity of Me(II)O is formed by0-10.0 wt. % of CaO, 0-7.5 wt. % of BaO, 0-9.0 wt. % of MgO, 0-3.5 wt. %of ZnO and 0-8.5 wt. % of SrO; and the quantity of Me(IV)O₂ is formed by0-5.0 wt. % of TiO₂ and 0-5.0 wt. % of ZrO₂.

Alkali-zinc-silicate glasses having the composition:

SiO₂ 52.0-63.5 wt. %, Me(III)₂O₃ 8.5-13.0 wt. %, K₂O 0-20.5 wt. %, Na₂O1.5-20.0 wt. %, Li₂O 0-5.0 wt. %, ZnO 2.0-8.0 wt. %, Me(II)O 2.5-6.5 wt.%, TiO₂+ZrO₂ 0.5-6.0 wt. %, SnO₂ 0-9.5 wt. %, P₂O₅ 0-4.0 wt. %, F 0-2.0wt. %, CeO₂ 0-3.0 wt. %, wherein the quantity of Me(III)₂O₃ is formed by0-13 wt. % of Al₂O₃ and 0-9.5 wt. % of La₂O₃; and the quantity ofMe(II)O is formed by 0-3.5 wt. % of CaO, 0-4.5 wt. % of BaO and 0-5.0wt. % of MgO.

In particular preference, however, at least one alkali silicate glasswhich can be produced by conventional methods having the followingcomposition 55.0-71.0 wt. % of SiO₂, 5.0-16.0 wt. % of Al₂O₃, 0.2-10.0wt. % of B₂O₃, 4.5-10.0 wt. % of K₂O, 3.0-14.0 wt. % of Na₂O, 0-4.0 wt.% of Li₂O, 0-3.0 wt. % of CaO, 0-5.0 wt. % of BaO, 0-4.0 wt. % of ZnO,0.2-5.0 wt. % of ZrO₂+TiO2, 0-2.0 wt. % of CeO₂, 0-3.0 wt. % of F and0-0.6 wt. % of P₂O₅ is used together with the apatite glass ceramic. Thewt. % details are based on the glass. Mixtures of the apatite glassceramic with at least one glass of this composition produce dentalmaterials which are particularly suitable as coatings for ceramicframeworks and hence for the production of fully ceramic dental productswith tooth-like optical properties and good chemical stability.

It is preferred to use glasses which do not crystallise during furtherprocessing of the dental material to dental products and particularlyduring sintering or other heating to 600° C. to 1000° C. for up to 2 h.Glasses having a sintering temperature from 650° C. to 1050° C. areadvantageous.

The dental material according to the invention is used preferably forcoating a substrate, particularly a dental crown or bridge. Inparticular, the dental material is sintered on to obtain the desiredcoating.

If used as a coating or veneering material, the apatite glass ceramic isusually comminuted initially to a powder with an average particle sizeof 5 to 80 μm, based on the number of particles. Additives such ascolour components and in particular glasses or further glass ceramics,and aqueous solutions for mixing or modelling, are optionally added tosaid powder, and the mixture obtained is applied to the substrate andshaped in the desired manner. After shaping, sintering finally takesplace at temperatures of 650° C. to 1050° C. to obtain the coated,shaped dental product.

It is also possible, however, to bond a dental restoration produced fromthe glass ceramic according to the invention to a substrate.

The apatite glass ceramic according to the invention may be used as acoating or veneering material for glass ceramic, all-ceramic or metallicdental frameworks or those based on a composite material, with a thermalexpansion coefficient of 7.0 to 12.0, particularly 8.0 to 11.0×10⁻⁶K⁻¹.It is used preferably for coating or veneering of ZrO₂ ceramics, Al₂O₃ceramics, ZrO₂/Al₂O₃ ceramics, ceramic or glass ceramic compositematerials and titanium.

It is used particularly advantageously, however, for veneeringframeworks based on lithium disilicate glass ceramic in order to producein this way aesthetically very attractive solid ceramic dental productswhich not only have excellent chemical stability but are alsocharacterised by very high strength.

Lithium disilicate glass ceramics which have proved to be particularlysuitable and were obtained by melting corresponding starting glasses,fritting and heat treatment at 400° C. to 1100° C. have the followingcomposition:

Component Wt. % SiO₂ 57.0 to 80.0 Al₂O₃   0 to 5.0 La₂O₃ 0.1 to 6.0 MgO  0 to 5.0 ZnO   0 to 8.0 K₂O   0 to 13.5 Li₂O 11.0 to 19.0 P₂O₅   0 to11.0

with the proviso that

(a) Al₂O₃+La₂O₃ is 0.1 to 7.0 wt. % and

(b) MgO+ZnO is 0.1 to 9.0 wt. %.

For the production of coatings, dental material according to theinvention having a thermal expansion coefficient that is smaller thanthat of the substrate to be coated is advantageous. Dental materialswhose expansion coefficient is not more than 3.0×10⁻⁶K⁻¹ smaller thanthat of the substrate are particularly advantageous. The dental materialpreferably has a linear thermal expansion coefficient of 5.5 to12.5×10⁻⁶K⁻¹, measured at temperatures from 100° C. to 400° C.

The apatite glass ceramic and the dental material according to theinvention may be processed in the usual way together with the additivesoptionally present to obtain shaped dental products. Suitable shapeddental products according to the invention containing the apatite glassceramic according to the invention or the dental material according tothe invention are, apart from compacts of the desired shape or ingots,particularly dental restorations such as an inlay, an onlay, a bridge,an abutment, a jacket, a veneer, a facet, a filling, a connector, acrown or a partial crown.

Preferred apatite glass ceramic according to the invention contains noleucite since, as a result of its high thermal expansion coefficient, itwould also confer a high thermal expansion coefficient of usually morethan 12.5×10⁻⁶K⁻¹ on the glass ceramic. If leucite-containing glassceramic is used to coat a substrate that has an expansion coefficient ofless than 12.5×10⁻⁶K⁻¹, such as ZrO₂ or lithium disilicate glassceramic, very high tensions are therefore also induced which result incracks and detachments.

The invention is explained in more detail below on the basis ofexamples.

EXAMPLES Example 1 to 17

A total of 17 different glass ceramics according to the invention wereproduced. They had the chemical compositions and molar ratios of CaO toP₂O₅ to F given in Table I and they all had a chemical stability of lessthan 100 μg/cm² loss of mass according to ISO 6872:1995.

TABLE 1 Composition of glass ceramics according to the invention(quantities in wt. %) and respective molar ratio CaO:P₂O₅:F Ex. Molarratio No. SiO₂ Al₂O₃ P₂O₅ CaO F K₂O Na₂O Li₂O B₂O₃ TiO₂ ZrO₂ CeO₂ BaOZnO SrO CaO:P₂O₅:F 1 64.5 8.4 1.1 2.8 0.7 6.6 9.6 — 2.2 1.2 0.4 — — 2.5— 1:0.156:0.719 2 56.9 19.7 2.6 1.7 2.5 8.0 8.6 — — — — — — — —1:0.6:4.2 3 45.2 21.9 3.2 4.5 0.6 7.5 13.0 0.4 2.0 0.5 0.7 0.5 — — —1:0.282:0.4 4 54.7 19.5 1.2 2.6 0.6 6.2 10.0 — 2.0 1.0 1.7 0.5 — — —1:0.172:0.641 5 62.0 9.0 2.6 4.3 0.6 7.5 8.6 — — 0.4 1.1 0.8 — — 3.11:0.24:0.42 6 64.4 6.3 1.2 2.8 0.6 6.5 9.8 — — 1.6 0.8 0.5 2.4 3.1 —1:0.172:0.656 7 69.7 5.1 3.7 5.3. 0.8 3.2 4.1 5.0 0.9 0.8 0.8 0.6 — — —1:0.277:0.429 8 62.8 13.1 1.2 2.7 0.6 6.3 5.9 — — — 1.7 0.5 1.8 3.4 —1:0.172:0.641 9 55.5 19.2 1.2 2.7 0.6 6.7 9.7 — 0.3 1.4 2.2 0.5 — — —1:0167:0.621 10 53.7 13.7 2.6 11.2 0.12 8.3 7.9 0.58 — 0.7 0.7 0.5 — — —1:0.086:0.032 11 55.5 9.8 2.8 4.9 1.5 5.7 3.8 0.2 8.0 1.3 1.1 0.5 2.82.1 — 1:0.224:0.897 12 55.4 19.9 0.5 2.0 0.4 7.4 7.3 — — — 1.6 0.7 4.8 —— 1:0.096:0.6 13 57.5 8.0 3.1 5.1 1.7 6.6 9.2 — 3.3 1.3 0.8 0.8 — 2.6 —1:0.241:1 14 59.2 7.9 3.0 5.1 0.6 6.8 9.6 0.3 1.0 1.5 2.5 0.5 — 2.0 —1:0.233:0.35 15 54.4 19.5 2.6 1.7 2.5 7.9 8.6 0.4 0.4 0.7 0.8 0.5 — — —1:0.6:4.2 16 54.3 18.4 6.4 1.7 2.4 7.8 7.5 0.4 0.4 0.7 — — — — —1:1.485:4.16 17 50.1 18.2 0.6 10.9 0.15 7.7 8.5 0.7 — 0.7 0.7 0.55 — 1.2— 1:0.021:0.04

In order to produce said glass ceramics, an appropriate batch ofsuitable oxides, carbonates and fluorides in each case was melted in aplatinum/rhodium crucible at a temperature of 1550° C. to 1600° C. for ahomogenisation period of 1 to 1.5 hours. The glass melt was quenched inwater, and the granules of starting glass formed were dried and groundto an average particle size of less than 90 μm.

The powder of starting glass obtained then underwent a heat treatment atmore than 900° C. and up to 1200° C. for 30 minutes to 6 hours,whereupon the glass ceramic formed.

Selected properties that were determined on specimens of the respectiveglass ceramic are given in Table II for some of the glass ceramics.Moreover, details about the heat treatment actually chosen for thestarting glass are given in Table II under “Heat treatment”.

The examples illustrate how, by altering the chemical composition, glassceramics with differing processing properties and optical properties butalways having good chemical stability can be obtained.

TABLE II Heat Firing α-value × Acid treat- temper 10⁻⁶K⁻¹ resist- mentature Tg (100° C.- Optical ance Ex. [° C./h] [° C.] [° C.] 400° C.)appearance [μg/cm²] 1 1050/1  860 545 8.4 milky, 21 slightly opal,translucent 8 1000/1 1080 650 7.9 very 23 translucent 9 1020/1 1050 6459.7 very 28 translucent 11 1000/1  890 547 6.6 yellowish, 58 milky,translucent 14 1050/1  870 541 9.4 whitish, 55 cloudy, translucent*Firing temperature = temperature which was used during production ofthe specimens by sintering onto quartz (1 minute: holding time, vacuum)

Determination of the expansion coefficient α

In order to measure the thermal expansion coefficient α, a rod-shapedgreen compact was prepared from powder of the glass ceramic in question,and said compact was sintered in a vacuum furnace at a rate of heatingof 60° C./min and with a holding time of 1 minute at the sinteringtemperature given in each case. A glaze bake was then carried outwithout vacuum at a 20° C. higher final temperature and with a holdingtime of 1 minute. The thermal expansion coefficient was determined onthe specimen obtained.

Determination of acid resistance

The acid resistance is a measure of the chemical stability of glassceramics used in the dentistry, since these are permanently exposed tothe action of acid substances in the oral cavity.

The acid resistance was determined according to the ISO specification6872:1995. To this end, small sample plates 12 mm in diameter and 1 mmthick were prepared initially by sintering together glass ceramicgranules with an average particle size of 90 μm. The granules were keptat the sintering temperature for 1 minute. The sample plates were thentreated for 16 hours in a Soxhlet apparatus with 4 vol. % of aqueousacetic acid and finally the loss of mass occurring was determined as ameasure of the acid resistance.

Mixtures of the apatite glass ceramics with additional components wereexamined in the following examples. Glasses and/or other glass ceramicswhich were used as additional components had the composition given inTable III.

TABLE III Composition of glasses and glass ceramics as additionalcomponents (details in wt. %) Add. com- ponent SiO₂ Al₂O₃ P₂O₅ CaO F K₂ONa₂O Li₂O B₂O₃ TiO₂ ZrO₂ CeO₂ BaO ZnO Alkali 61.5 8.7 — 1.0 1.7 7.0 8.8— 2.4 1.5 1.0 0.5 2.9 3.0 silicate glass (A) Alkali 61.4 8.5 — 1.1 1.77.8 8:7 0.6 1.9 1.5 1.0 0.5 2.1 3.2 silicate glass (B) Alkali 62.3 8.7 —1.3 1.6 7.0 7.0 2.0 1.1 1.4 1.0 0.6 3.0 3.0 silicate glass (C) Alkali70.8 8.6 — 2.1 0.9 6.9 8.3 1.5 0.2 0.7 — — — — silicate glass (D) Alkali63.4 6.2 0.4 1.7 — 6.4 9.6 — 3.7 1.7 1.1 0.5 2.3 3.0 silicate glass (E)Alkali 61.9 9.9 — 1.1 1.5 5.8 3.7 0.2 8.0 1.4 1.1 0.5 2.8 2.1 silicateglass (F) Leucite 56.8 13.6 2.6 3.7 0.3 10.8 7.5 0.2 0.3 0.4 1.1 1.0 0.90.8 phos- phosi- licate glass ceramic (G) Alkali 57.1 9.5 — 1.9 0.9 9.69.3 1.7 — — 1.0 1.0 3.9 4.1 zinc silicate glass (H) Opal- 55.8 15.2 2.62.6 — 11.0 9.6 — 0.3 — 1.9 1.0 — — escent glass (I)

Example 18

This Example describes the use of the glass ceramic according to theinvention according to Example 9 as a coating material for ceramicframeworks and thus for the production of all-ceramic dental products.

Glass powder of the appropriate composition was heat treated for 1 hourat 1020° C. for the production of the glass ceramic. The glass ceramicformed was examined by scanning electron microscopy and the crystalsformed could be identified by X-ray diffractometry as needle-shapedapatite crystals.

In order to obtain a suitable expansion coefficient and sinteringtemperature, the glass ceramic was mixed with the alkali silicateglasses (A) and (B) (see Table III).

The production of these alkali silicate glasses took place in a mannersimilar to the production of the starting glasses described above inExamples 1 to 17.

The glass ceramic and the two alkali silicate glasses were mixed in theform of powders having an average particle size of less than 90 μm andin a weight ratio of 40% apatite glass ceramic according to Example 9(see Table II), 30% alkali silicate glass (A) and 30% alkali silicateglass (B).

This mixture was sintered at 870° C. to a rod-shaped green compact in avacuum furnace at a rate of heating of 60° C./min and with a holdingtime of 1 min. A thermal expansion coefficient of 9.5×10⁻⁶K⁻¹, measuredin the temperature range of from 100 to 400° C., was determined for thesample obtained.

This mixture could thus be used for sintering on to a substrate with athermal expansion coefficient of 10.6×10⁻⁶K⁻¹, such as lithiumdisilicate glass ceramic, at an advantageous processing temperature of830° C. This processing on the tooth substrate can usually take place attemperatures that are 50° C. to 100° C. lower than for sintering ontoquartz.

The fully ceramic products obtained are characterised by good chemicalstability, an aesthetic appearance and high strength.

Example 19

In the same way as Example 18, different apatite glass ceramicsaccording to the invention may also be mixed together or with otherglasses to obtain desired expansion coefficients and sinteringtemperatures.

A powder mixture of 25 wt. % of glass ceramic according to Example 4(heat treatment at 1020° C.), 50 wt. % of glass ceramic according toExample 14 (heat treatment at 1050° C.) and 25 wt. % of alkali silicateglass (B) (see Table III.) was produced. This mixture had anadvantageous sintering temperature of only 830° C. and an expansioncoefficient of 9.5×10⁻⁶K⁻¹.

The mixture had good chemical stability and outstanding opticalproperties and was highly suitable as a sintering ceramic for anall-ceramic dental framework with a low expansion coefficient.

Example 20 to 27

Further mixtures of apatite glass ceramics according to the inventionwith glasses and glass ceramics were examined in these Examples.

The compositions of the individual mixtures and the heat treatmentcarried out for the production of the apatite glass ceramic used in eachcase are listed in Table IV.

The properties determined for these mixtures are also give in Table IV,and they show that it is possible, by means of a suitable choice ofcomponents, to obtain dental materials with properties matched to theapplication in question which in any event are-characterised by goodchemical stability.

TABLE IV Compositions and properties of mixtures of apatite glassceramics according to the invention with glasses and/or glass ceramicsα-value × Heat Firing 10⁻⁶k⁻¹ Acid treatment Mixing ratio temp Tg (100°C.- Optical resistance Ex. Composition [° C./h] [in wt. %] [° C.] [° C.]400° C.) appearance [μg/cm²] 20 Apatite glass ceramic 9 1000/1 30 880528 9.5 very 34 Alkali silicate glass (A) — 35 translucent Alkalisilicate glass (B) — 35 21 Apatite glass ceramic 14 1050/1 50 850 5309.3 milky cloudy, 38 Alkali silicate glass (A) — 50 translucent 22Apatite glass ceramic 14 1020/1 50 870 542 8.0 milky <100 Alkalisilicate glass (F) — 50 translucent 23 Apatite glass ceramic 8 1000/1 40910 552 8.8 very 29 Alkali silicate glass (C) — 60 translucent 24Apatite glass ceramic 1 1050/1 70 850 539 8.7 slightly milky, 26 Alkalisilicate glass (D) — 30 slightly opal, translucent 25 Apatite glassceramic 9 1020/1 20 780 463 10.9 transparent 24 Alkali zinc silicateglass — 80 (H) 26 Apatite glass ceramic 9 1020/1 50 1020  600 10.2 verytrans- 27 Opalescent glass (J) — 50 lucent, slightly brownish opal 27Apatite glass ceramic 14 1050/1 30 910 560 9.7 whitish, 45 Leucitephosphosilicate 70 translucent glass ceramic (G)

Example 28

In this Example, the translucence was determined quantitatively bydetermining the CR value of selected dental materials according to theinvention.

The British Standards Institution method of measurement was used forthis purpose, which is described in the test standard for dental ceramic“BS 5612:1978”.

Five specimens per material with a diameter of 20 mm and a samplethickness of 1.75 mm were fired at an appropriate sintering temperature.The specimens were ground with wet SiC powder, grain size 320, in orderto obtain the desired surface quality (surface roughness Ra=0.8 μm-1.6μm). It is important that the plane-parallelism of the opposite sidesdoes not exceed a tolerance of ±0.01 mm since the measuring resultdepends to a large extent on the layer thickness. The final sampleheight/thickness should be 1.00±0.025 mm.

The specimens were placed in the designated opening in a Minolta-CR 300colour measuring instrument and the reflection of each of the 5specimens was measured with an aperture of 10 mm. The samples must notbe in optical contact with the background during the measurement, asituation which may be prevented if necessary by applying a drop ofglycerol onto the background.

(a) In order to determine the sample emission on a black backgroundY_(b) (Y_(black)), a black plate with not more than 4% reflection wasused.

(b) In order to determine the sample emission on a white backgroundY_(w) (Y_(white)), a white plate with a reflection of 80 to 85% wasused.

The contrast value CR was then calculated from the Y_(b) and Y_(w)values determined according to CR=Y_(b)/Y_(w), and it was as follows forthe two materials examined:

Material 1: CR₁=0.13→13% opacity

Material 2: CR₂=0.50→50% opacity

The materials had the following composition:

Material 1: Composition like the mixture according to Example 20

Material 2: 50 wt. % of mixture according to Example 20 50 wt. % ofapatite glass ceramic according to Example 14 (heat treatment 1050° C.,1 hour)

The above results show that the translucence can be adjusted by asuitable choice of the composition of the materials.

What is claimed is:
 1. A dental material comprising a chemically stable,translucent apatite glass ceramic, which contains CaO, P₂O₅ and F in amolar ratio of CaO:P₂O₅:F 1:0.020 to 1.5:0.03 to 4.2 and comprisesapatite crystals as the main crystal phase, wherein the apatite crystalsare needle-shaped.
 2. A dental material according to claim 1, whereinthe apatite glass ceramic contains CaO, P₂O₅ and F in a molar ratio ofCaO:P₂O₅:F which is 1:0.1 to 0.5:0.1 to 1.0.
 3. A dental materialaccording to claim 1, wherein the apatite glass ceramic has a loss ofmass, determined according to ISO specification 6872:1995, of less than200 μg/cm².
 4. A dental material according to claim 1, wherein theapatite glass ceramic comprises the following components: Component Wt.% SiO₂ 45.0 to 70.0 Al₂O₃  5.0 to 22.0 P₂O₅ 0.5 to 6.5 K₂O 3.0 to 8.5Na₂O  4.0 to 13.0 CaO  1.5 to 11.0 F  0.1 to 2.5.


5. A dental material according to claim 4, wherein the apatite glassceramic further comprises at least one of the following components:Component Wt. % B₂O₃ 0 to 8.0 La₂O₃ 0 to 5.0 Li₂O 0 to 5.0 BaO 0 to 5.0MgO 0 to 5.0 ZnO 0 to 5.0 SrO 0 to 7.0 TiO₂ 0 to 4.0 ZrO₂ 0 to 4.0 CeO₂ 0 to 3.0.


6. A dental material according to claim 4, wherein the apatite crystalsare smaller than 35 μm in their greatest dimension.
 7. A dental materialaccording to claim 1, wherein the apatite glass ceramic has a thermalexpansion coefficient of 6.0 to 12.0×10⁻⁶K⁻¹, measured in thetemperature range of from 100 to 400° C.
 8. A dental material accordingto claim 1, wherein the apatite glass ceramic has a CR value of 0 to0.9.
 9. A dental material according to claim 1 further comprising atleast one glass and/or glass ceramic of the systems selected from thegroup consisting of alkali silicate, alkali-alkaline earth-silicate,alkali-aluminosilicate, alkali-zinc-borosilicate, phosphosilicate andalumino-fluoro-borosilicate.
 10. A dental material according to claim 1,wherein the apatite glass ceramic has a thermal expansion coefficient of5.5 to 12.5×10⁻⁶ K⁻¹, measured in the temperature range of from 100 to400° C.
 11. A dental material according to claim 3, wherein the apatiteglass ceramic has a loss mass, determined according to ISO specification6872: 1995, of less than 100 μcm².
 12. A shaped dental product, whichcomprises the dental material according to claim
 1. 13. A shaped dentalproduct according to claim 12, which is a dental restoration.
 14. Ashaped dental product according to claim 12, which comprises a corecomprising ceramic or glass ceramic material and a coating applied tothe core, wherein the coating comprises the apatite glass ceramic or thedental material.
 15. A shaped dental product according to claim 14,wherein the glass ceramic material is a lithium disilicate glassceramic.
 16. A process for the preparation of the apatite glass ceramicaccording to claim 1, said process comprising a) melting a startingglass comprising the necessary components at temperatures of 1200° C. to1650° C., b) pouring the glass melt into water to form glass granules,c) optionally comminuting the glass granules to a glass powder with anaverage particle size of 1 to 450 μm, based on the number of particles,and d) subjecting the glass granules or the glass powder to a treatmentat temperatures of more than 900° C. and up to 1200° C. for a period of30 minutes to 6 hours.
 17. A method of coating a substrate comprisingproviding the substrate and coating the substrate with the dentalmaterial of claim
 1. 18. The method according to claim 17, wherein thesubstrate is comprised of a ceramic or glass ceramic.
 19. The methodaccording to claim 18, wherein the substrate is comprised of lithiumdisilicate glass ceramic.
 20. The method according to claim 19, whereinthe lithium disilicate glass ceramic comprises: Component Wt. % SiO₂57.0 to 80.0 Al₂O₃   0 to 5.0 La₂O₃ 0.1 to 6.0 MgO   0 to 5.0 ZnO   0 to8.0 K₂O   0 to 13.5 Li₂O 11.0 to 19.0 P₂O₅   0 to 11.0

with the proviso that (a) Al₂O₃+La₂O₃ is 0.1 to 7.0 wt. % and (b)MgO+ZnO is 0.1 to 9.0 wt. %.
 21. The method according to claim 17,further comprising sintering the coated substrate at temperatures of650° C. to 1050° C.